Thermal printers work by selectively heating regions of special heat-sensitive paper. A thermal print head has a line of resistors, whose output is a line of precisely controlled dots of heat, which produce images in conjunction with heat-sensitive paper or ribbons.
A typical thermal printer traditionally has a thermal head (to generate heat and print on paper), a platen (a rubber roller that feeds paper), a spring (which applies pressure to the thermal head, causing it to contact thermo-sensitive paper), and at least one of a controller board (for controlling the mechanism). In order to print, one inserts thermo-sensitive paper between the thermal head and the platen. The printer sends an electrical current to the heating resistor of the thermal head which in turn generates heat in a prescribed pattern. The heat activates the thermo-sensitive coloring layer of the thermo-sensitive paper, which manifests a pattern of color change in response.
The paper is impregnated with a solid-state mixture of a dye and a suitable matrix, e.g. a combination of a fluoran leuco dye and an octadecylphosphonic acid. When the matrix is heated above its melting point, the dye reacts with the acid, shifts to its colored form, and the changed form is then conserved in meta-stable state when the matrix solidifies back quickly enough.
Controller boards are embedded with firmware to manage the thermal printer mechanisms. These controller boards' features are designed to meet the needs in terms of functionalities and specifications.
The firmware can manage multiple code types, graphics and logos. This enables a user to choose between different resident fonts and character sizes.
Controller boards can drive various sensors like paper low, paper out, door open, top of form etc., and they are available with the most commonly used interfaces (RS232, Parallel, USB, wireless).
Thermal print heads require a heat sink to dissipate the heat generated during printing, thus adding to manufacturing costs.
Most thermal printer applications employ a thermal print head having a linear array of thermal print cells (dots). In such application, either the thermal print head is held stationary and the thermal-sensitive paper is moved, or the paper is held stationary and the thermal print head is moved; one in contact with the other, at a controlled velocity and/or with feed-back to the controlling system indicating position and/or velocity while the controller selectively energizes the dots of the thermal print head to mark the paper passing against it.
Both of these situations are impractical for time & attendance applications where an employee will present his/her time card to the printing time clock manually. Unlike printers using roll-paper, a time clock with fixed print-head cannot precisely control the movement of a manually inserted time card without resorting to motion-control components of significant cost. Utilizing a moving print-head would require an active clamping mechanism to keep the time card stationary and a complex and costly means to precisely transport the print-head. By utilizing a multi-segment/multi-character fixed thermal print head, the number of mechanical and motion-control components is minimized, the need for precise motion control is eliminated, and the precisely-timed activation of thermal print segments can be easily and inexpensively controlled using current microcontroller technology.
U.S. patents directed to thermal print heads include the following.
U.S. Pat. No. 3,934,695, issued to Kovalick on Jan. 27, 1976, discloses the enhancement of the quality of thermally printed characters by controlling the time at which and the time for which power is applied to the resistive printing elements in a battery-operated moving-head thermal dot matrix printer. By sequentially strobing the elements in the pattern of the character to be formed as the print head moves across thermal sensitive paper, a high-quality slanted character is printed and parasitic losses are reduced. By inversely varying the time power is supplied to each dot as battery voltage varies, character quality is maintained and useful battery life is extended
U.S. Pat. No. 4,262,188, issued to Beach on Apr. 14, 1981, discloses enhancing the uniformity of density of characters printed by thermal printers upon thermally sensitive paper by controlling the amount of energy supplied to the print head during subsequent printings before the print head has completely cooled to ambient temperature. To obtain the desired uniformity the energy supplied to the print head for subsequent printings is made proportional to the energy lost by cooling of the print head between printings. This results in the print head being reheated to substantially the same printing temperature for each printing of a character or character segment. By using a dot driver having an R-C circuit that recharges the capacitor between print pulses at a rate that is proportional to the thermal time constant of the print head, the energy stored by the capacitor can then be used to re-heat, or control the re-heating, of the print head to substantially the same selected print temperature. By maintaining the R-C charging time constant substantially between 0.1.tau. and .tau. (.tau. is the thermal time constant of the print head) the resultant printed character segments have substantially uniform density.
U.S. Pat. No. 4,475,112, issued to Washio et al. on Oct. 2, 1984, discloses a thermal printing head comprising an array of heating elements divided into two blocks each having alternate pairs of two adjacent heating elements, the two blocks being further divided into eight subblocks. The pairs of two adjacent heating elements are supplied with electric power through power feed lines each shared by such a pair of two adjacent heating elements. Two adjacent heating elements belonging to the two blocks are drivable by a single driver. Therefore, the number of the power feed lines and the drivers can be reduced. The eight subblocks are driven two at a time in one cycle of operation, and hence can be energized by a limited allowable current supplied to the thermal printing head.
U.S. Pat. No. 4,777,583, issued to Minami et al. on Oct. 11, 1988, discloses a thermal head comprising a ceramic substrate, a glaze layer partially formed on the ceramic substrate, heat-generating resistors and electrodes connected to both the ends of the heat-generating resistors, if the width of individual electrodes located outside the glaze layer is made narrower than the width of corresponding electrodes on the glaze layer, formation of a short circuit between adjacent individual electrodes because of the presence of voids on the ceramic substrate can be effectively prevented.
U.S. Pat. No. 4,789,870, issued to Lacord et al. on Dec. 6, 1988 discloses a thermal series type printing head that is controlled on-the-fly, during a succession of cycles of a duration at most equal to the time for printing the points, so as to be heated independently during the first half and during the second half of each cycle. Thus an offset dot can be printed of half a length of a normal dot, or an extended dot, one and half times as long as the normal dot, for improving the definition of printing without reducing the writing speed. The invention applies to printing systems, particularly for printers connected to word processing devices.
U.S. Pat. No. 4,861,625, issued to Kondo et al. on Aug. 29, 1989 discloses a method of manufacturing a partially-glazed ceramic substrate for use in a thermal printing head. A ceramic substrate having a surface roughness of 0.2 .mu.m or less is provided. Subsequently, a glaze is applied to the ceramic substrate to form raised glaze regions having a transverse width of 1.0 mm or less and thickness of 100 .mu.m or less. The substrate and glaze are baked, and then a heating element is formed on the raised glaze regions.
U.S. Pat. No. 4,944,983, issued to Nonoyama et al. on Jul. 31, 1990 discloses a sloped substrate for a thermal head made of ceramic for a thermal head of a thermosensitive printing device, in which a sloped surface of 200 .mu.m to 2,000 .mu.m in width is formed between a main plane surface of the substrate and a subplane surface thereof and a glaze is bonded by firing to the main plane and the subplane surfaces and the sloped surface so that the thickness of the glaze is 100 .mu.m or less.
U.S. Pat. No. 5,514,524, issued to Ohnishi et al. on May 7, 1996, discloses a method of making thermal printheads is provided which comprises the steps of: (a) preparing a master substrate having plural rows of unit head regions; (b) forming a head glaze member in each unit head region in each row so that an edge of the head glaze member of the unit head region aligned with that of the head glaze member of any other unit region in the same row; (c) half-cutting the master substrate along the edge of the head glaze member of the unit head region with a half-cutting dicing blade which has an inclined edge face for partially cutting the head glaze member to provide a glaze corner; and (d) forming an array of heating dots along the glaze corner; wherein at least one blade positioning mark is formed on the master substrate before the half-cutting step (c); and the half-cutting dicing blade is positionally set in the half-cutting step (c) by referring to the blade positioning mark.
U.S. Pat. No. 5,519,426, issued to Lokis et al. on May 21, 1996, discloses a method for controlling binary thermal printers which increases the effective output resolution of the thermal printer above the native resolution of a print head having a plurality of individual resistive heating elements arranged in a print line. An increase in the effective resolution of a binary output image is achieved by using an overdrive energy to control a relative position of a binary edge of a pixel image at a resolution that is less than the native resolution of the thermal printer. In a preferred embodiment, an under-drive energy may also be used with an adjacent over-drive energy to further control the relative position of the binary image of the pixel image. The over-drive energy is higher than a native pixel drive energy, but lower than a maximum drive energy. The native pixel drive energy produces a binary pixel image having a native area corresponding to the native resolution of the thermal printer. The binary pixel image on the print media corresponding to the heating elements to which the over-drive energy is applied are increased in area beyond the native area of the thermal printer, thereby enabling the thermal printer to realize an increase in an effective resolution of the binary image.
U.S. Pat. No. 5,995,127, issued to Uzuka on Nov. 30, 1999, discloses a thermal print head with a supporting substrate, a glaze layer formed on the substrate, a heating resistor which is formed on the glaze layer and made of Si and O and the rest being substantially composed of a metal, and electrodes connected to the heating resistor. The heating resistor has an unpaired electron density of 1.0.times.10.sup.19/cm.sup.3. In addition, the reaction layer formed by reaction of the glaze layer and the heating resistor is formed between the glaze layer and resistor.
U.S. Pat. No. 6,030,071, issued to Komplin et al. on Feb. 29, 2000, discloses a printhead comprising a plate having a plurality of orifices through which ink droplets are ejected and a heater chip coupled to the plate. The heater chip includes a plurality of heating elements and first and second conductors for providing energy to the heating elements. The first and second conductors are arranged in spaced-apart planes and/or in a matrix.
U.S. Pat. No. 6,081,287, issued to Nosita et al. on Jun. 27, 2000, discloses a thermal head having a protective film of a heater formed on the heater, the protective film comprising a ceramic-based lower protective layer composed of at least one sub-layer and a carbon-based upper protective layer formed on the lower protective layer, wherein a surface of the lower protective layer on which the upper protective layer is to be formed has a surface roughness value Ra of 0.005 to 0.5 .mu.m; or the one in which the depth of a depression step which may be formed on the surface of the lower protective layer due to the thickness of the electrodes used for supplying power to the heater (or heat-generating resistor) was reduced to 0.2 .mu.m or less. Therefore, the thermal head has a protective film which has significantly reduced corrosion and wear, which is advantageously protected from cracks and peeling-off due to heat and mechanical impact and which allows the thermal head to have a sufficient durability to exhibit high reliability over an extended period of time, thereby ensuring that the thermal recording of high-quality images is consistently performed over an extended period of operation.
U.S. Pat. No. 6,344,868, issued to Susukida et al. on Feb. 5, 2002, discloses a thermal head including a protection layer having mutually opposed first and second surfaces, the first surface having a flat or protruded printing surface which is brought into contact with a heat sensitive record medium, a heat generating section including resistors and electrodes connected to the electrodes and provided on the second surface of the protection layer, and a reinforcing member made of a low melting pint glass and provided on a side of the heat generating section remote from the protection layer. The reinforcing member improves a mechanical strength of the thermal head. The reinforcing member made of a glass also serves as a heat storage member, and thus a thermal property of the thermal head is improved. The reinforcing member may be formed by an aggregate of ceramic particles. The reinforcing member may contain a heat storage layer made of a low melting point glass and a heat conduction layer provided on the heat storage layer.
U.S. Pat. No. 6,558,563, issued to Kashiwaya et al. on May 6, 2003, discloses a thermal head fabricating method which forms a lower protective layer made of ceramics for protecting a plurality of heat-generating resistors and electrodes, subjects the lower protective layer to etching processing by a plasma and forms a carbon protective layer on the thus subjected lower protective layer. The etching processing is performed using a mask which defines an area where the carbon protective layer is formed, a protective layer is formed on a surface of the mask, and the protective layer is made of a material which is etched at an extremely slow rate or substantially not etched compared with ceramics composing the lower protective layer and/or which does not impart an adverse effect to the carbon protective layer that is subsequently formed.
U.S. Pat. No. 6,614,460, issued to Susukida et al. on Sep. 2, 2003, discloses a thermal head including a protection layer having mutually opposed first and second surfaces, the first surface having a flat or protruded printing surface which is brought into contact with a heat sensitive record medium, a heat generating section including resistors and electrodes connected to the electrodes and provided on the second surface of the protection layer, and a reinforcing member made of a low melting pint glass and provided on a side of the heat generating section remote from the protection layer. The reinforcing member improves a mechanical strength of the thermal head. The reinforcing member made of a glass also serves as a heat storage member, and thus a thermal property of the thermal head is improved. The reinforcing member may be formed by an aggregate of ceramic particles. The reinforcing member may contain a heat storage layer made of a low melting point glass and a heat conduction layer provided on the heat storage layer.
While these patents and other previous methods have attempted to solve the problems that they addressed, none have utilized or disclosed a multi-segment, multi-character thermal print head assembly and energizing schema which eliminates the need for a heat-sink.
Therefore, a need exists for a solution to the above problems. The attributes and functionalities of the technology described herein provide this solution. The print head assembly according to embodiments of the invention substantially departs from the conventional concepts and designs of the prior art. It can be appreciated that there exists a continuing need for a new and improved print head assembly which can be used commercially. In this regard, the technology described herein substantially fulfills these objectives.
The foregoing information reflects the state of the art of which the inventors are aware and is tendered with a view toward discharging the inventors' acknowledged duty of candor in disclosing information that may be pertinent to the patentability of the technology described herein. It is respectfully stipulated, however, that the foregoing information do not teach or render obvious, singly or when considered in combination, the inventors' claimed invention.